How to use the Otto & Diesel Cycle Calculator
- 1. Pick the cycleOtto (petrol), Diesel or Dual (mixed) cycle.
- 2. Enter the ratiosCompression ratio r, γ, and cut-off ratio ρ or pressure ratio α.
- 3. Add start stateT1, p1 and (Otto) heat added, to get every state, net work and MEP.
Efficiency formulas
- Otto: η = 1 − 1 / rγ−1
- Diesel: η = 1 − (1 / rγ−1) × (ργ − 1) / [γ(ρ − 1)]
- Dual: η = 1 − (1 / rγ−1) × (αργ − 1) / [(α − 1) + αγ(ρ − 1)]
The states follow from isentropic relations T2 = T1·rγ−1 and p2 = p1·rγ. Net work = heat in − heat out, and MEP = net work / swept volume (v1 − v2), where v1 = RT1 / p1.
Worked example and exam tips
Otto cycle, r = 8, γ = 1.4: η = 1 − 1 / 80.4 = 1 − 1 / 2.297 = 56.5%. Diesel cycle, r = 16, ρ = 2: η = 1 − (1 / 160.4) × (21.4 − 1) / (1.4 × 1) = 61.4%.
- For the same compression ratio, Otto is more efficient than Diesel. Diesel engines win because they run at much higher r.
- As the cut-off ratio grows, Diesel efficiency falls.
- The Dual cycle sits between Otto and Diesel, and becomes Diesel when α = 1.
Frequently asked questions
What is the cut-off ratio?
ρ = v3 / v2, the ratio of volume at the end of heat addition to the volume at the end of compression in a Diesel cycle.
Why is real engine efficiency lower?
Real engines have heat loss, friction, incomplete combustion and variable specific heats. The air-standard cycle is an ideal upper limit.
What does MEP tell me?
Mean effective pressure is the constant pressure that would give the same net work over one stroke. Higher MEP means more work from the same engine size.
Is this calculator free?
Yes. You enter your name and WhatsApp number once to see your result, and all other Uncodemy tools then open without the form. Everything is calculated in your browser.